diff --git a/tools/chibi-ffi b/tools/chibi-ffi
index 0b73c636..06387ba2 100755
--- a/tools/chibi-ffi
+++ b/tools/chibi-ffi
@@ -1509,6 +1509,8 @@
       (if (pair? (struct-fields type))
           (let ((len (make-integer (length (struct-fields type)))))
             (cat "  sexp_type_slots(" (type-id-name name) ") = SEXP_NULL;\n"
+                 "  sexp_type_field_len_base(" (type-id-name name) ")"
+                 " = " (length (struct-fields type)) ";\n"
                  (lambda ()
                    (do ((ls (reverse (struct-fields type)) (cdr ls)))
                        ((not (pair? ls)))
diff --git a/vm.c b/vm.c
index fdb4bce9..08fa331e 100644
--- a/vm.c
+++ b/vm.c
@@ -1537,6 +1537,8 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
         if (sexp_car(tmp1) == _ARG3) { _ARG3 = sexp_make_fixnum(i); break; }
     if (! sexp_fixnump(_ARG3))
       sexp_raise("slotn-ref: not an integer", sexp_list1(ctx, _ARG3));
+    if (sexp_unbox_fixnum(_ARG3) < 0 || sexp_unbox_fixnum(_ARG3) >= sexp_type_field_len_base(_ARG1))
+      sexp_raise("slotn-ref: slot out of bounds", sexp_list2(ctx, _ARG3, sexp_make_fixnum(sexp_type_field_len_base(_ARG1))));
     if (sexp_vectorp(sexp_type_getters(_ARG1))) {
       tmp1 = sexp_vector_ref(sexp_type_getters(_ARG1), _ARG3);
       if (sexp_opcodep(tmp1))
@@ -1561,12 +1563,14 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
         if (sexp_car(tmp1) == _ARG3) { _ARG3 = sexp_make_fixnum(i); break; }
     if (! sexp_fixnump(_ARG3))
       sexp_raise("slotn-set!: not an integer", sexp_list1(ctx, _ARG3));
+    if (sexp_unbox_fixnum(_ARG3) < 0 || sexp_unbox_fixnum(_ARG3) >= sexp_type_field_len_base(_ARG1))
+      sexp_raise("slotn-set!: slot out of bounds", sexp_list2(ctx, _ARG3, sexp_make_fixnum(sexp_type_field_len_base(_ARG1))));
     if (sexp_vectorp(sexp_type_setters(_ARG1))) {
       tmp1 = sexp_vector_ref(sexp_type_setters(_ARG1), _ARG3);
       if (sexp_opcodep(tmp1))
         _ARG4 = ((sexp_proc3)sexp_opcode_func(tmp1))(ctx, tmp1, 2, _ARG2, _ARG4);
       else
-        sexp_raise("slotn-ref: no setter defined", sexp_list1(ctx, _ARG3));
+        sexp_raise("slotn-set!: no setter defined", sexp_list1(ctx, _ARG3));
     } else
       sexp_slot_set(_ARG2, sexp_unbox_fixnum(_ARG3), _ARG4);
     top-=4;